"Io ho" or just "ho"?
I have seen both for "I have", so I'd like to know when it's necessary to have "Io" before the verb, to have (ho).
It is not necessary, since "avete" only becomes "ho" with "io". It is not like English wherein you can use have for i, you, we and they.
Thanks, so the same (omission of the pronoun) can be applied to the others (e.g. Mangiamo without noi, Mangi without tu, and Mangia without Lui/Lei?)
Speaking the subject pronoun or not is a free choice, but in most cases it is unspoken.
The only cases when the subject pronoun must be spoken are:
(1) when the subject pronoun is emphasized:
Noi mangiamo a casa. = We (not you nor anyone else) eat / are eating at home.
Lui ha la mia penna. = He (not me nor anyone else) has my pen.
(2) when the same verb takes more than one subject:
- tu e Paolo avete lo stesso cellulare. = You and Paul have the same cell phone.
Tu e lui lavorate in un ufficio. = You and he work in an office.
(3) when the sentence includes two clauses with different subjects, so the subject pronoun automatically sounds as opposed to the other subject (thus slightly emphasized):
Noi parliamo in italiano ma loro non capiscono. = We speak in Italian but they don't understand.
Io viaggio in autobus e Maria prende sempre il treno. = I travel by bus and Mary always takes the train.
Both are correct. Io could be omitted. So it is not wrong to say "io ho" or only "ho"
Just to add to what our friends have written. Sometimes you would still use 'io' (and other personal pronouns) before a verb, especially to emphasise something.
For instance, if someone where to say I have not got a cat, your could say 'io ho un gatto' basically saying well I have!